docs(orderBy): clarify expression usage in a predicate

Closes angular#8592

Author: pkozlowski-opensource

src/ng/filter/orderBy.js

@@ -18,9 +18,13 @@
  *
  *    - `function`: Getter function. The result of this function will be sorted using the
  *      `<`, `=`, `>` operator.
- *    - `string`: An Angular expression which evaluates to an object to order by, such as 'name'
- *      to sort by a property called 'name'. Optionally prefixed with `+` or `-` to control
- *      ascending or descending sort order (for example, +name or -name).
+ *    - `string`: An Angular expression. The result of this expression is used to compare elements
+ *      (for example `name` to sort by a property called `name` or `name.substr(0, 3)` to sort by
+ *      3 first characters of a property called `name`). The result of a constant expression
+ *      is interpreted as a property name to be used in comparisons (for example `"special name"`
+ *      to sort object by the value of their `special name` property). An expression can be
+ *      optionally prefixed with `+` or `-` to control ascending or descending sort order
+ *      (for example, `+name` or `-name`).
  *    - `Array`: An array of function or string predicates. The first predicate in the array
  *      is used for sorting, but when two items are equivalent, the next predicate is used.
  *